Liszt...
I don't think there is any question, even though none of us ever heard him perform. The man could read anything at sight almost flawlessly, had massive technique (evident from his compositions), was in-tune with the modern music of his day, and well ahead in his own compositions (ie. B minor sonata contains the first 12 tone row) which makes him an even greater musician. Plus he was good looking, charming, and had the showmanship to boot! Also, it is evident from his compositions that he was not only a technical player, but one with superior musical intellect and expression. I don't know of any other pianist who had all of these attributes to such a great extent. Of course I do not mean to say that there are/were no other great pianists...but come on guys...Liszt is the God of the piano.